Proteomics method for identifying POT1-associated complexes at telomeres using ChIP-Mass spectrometry

POT1 is the only single stranded telomere binding protein in the shelterin complex. Together with TPP1, POT1 plays a crucial role in regulating telomere length and protecting telomeres from DNA damage repair proteins. The activation of DNA damage repair proteins at telomeres can be detrimental to cells, so their activity must be suppressed. POT1 interacts with other telomeric proteins (TRF2, TRF1, TIN2 and RAP1) via its association with TPP1. These proteins function together to protect and maintain the telomeres. Despite extensive knowledge of POT1's role within the shelterin complex, the full spectrum of its interactors at the single-stranded telomeric overhang remains poorly defined. To study these interactions, we generated an endogenous Flag-tag knock-in of POT1 using the CRISPR-Cas9 gene editing system. To address the risk of unintended gene disruption associated with this technique, we conducted an in-depth characterization of the endogenously Flag-tagged POT1 clone to ensure that its telomere and TPP1 binding functions remained intact. Further, we performed proteomic profiling of the Flag-tagged POT1 within the chromatin fraction using ChIP-MS to explore its proteome. Our analysis uncovered a novel set of POT1-associated proteins at the extremes of telomeres. Given that POT1 exclusively binds to the single-stranded 3' overhang of telomeres, the proteomic data obtained indicates POT1 interactions occurring at the extreme ends of telomeres. In conclusion, our study reveals previously uncharacterized POT1 associated proteins using ChIP mass spectrometric approach, paving the way for further investigations into telomere biology and potential therapies targeting telomere regulation.